A 2-GHz GaAs HBT RF Pulsewidth Modulator

RF pulsewidth modulation (PWM) has been proposed as an efficiency improving technique for power amplifiers (PAs) handling varying envelope signals. The core idea is to modulate the varying envelope into a square-wave signal such that the width of each pulse is varied according to the envelope, and any phase information is contained in the timing of the pulses. The square wave signal can give a theoretical efficiency of 100% for the subsequent PA. Such a system differs significantly from a conventional up-conversion and PA and, therefore, a significant design challenge results. The first important step is to generate the PWM signal. This paper demonstrates a functional modulator for Universal Mobile Telecommunications System operating at 2 GHz. The modulator is designed in a GaAs HBT process and uses low-frequency feedback combined with predistortion to obtain a high spectral purity. Measurements show a Universal Mobile Telecommunications System PWM signal with more than 10-dB margin to the modulation mask and an error vector magnitude of less than 1.5% rms (requirement: 17.5%).